This invention relates to apparatus for working on mounting grooves formed at bases of blades of a turbine rotor, and, more particularly, to an apparatus for providing mounting grooves of fine finishes by machining radial entry type blades fitted to the turbine rotor from the radial direction.
Turbine blades rotate at high speed in high temperature atmosphere. In fitting the blades to a turbine rotor shaft, the following process has generally been in use. The blades are each formed at the base with a mounting groove in the form of a Christmas tree, and disks integral with the turbine rotor shaft are each formed at the top with a protrusion of a cross-sectional shape conforming to the mounting groove of each blade. The disks are each cut out in one portion to allow the mounting grooves of the blades to be fitted over the protrusion of the disk through the cutout. The blades thus mounted on the disk are successively moved circumferentially of the disk so that they are securely fitted to the disk in the form of a torus. This process is generally referred to as a radial entry process. In mounting the blades on the disks, it is essential that the mounting grooves of the blades and the protrusion of the disk of the rotor shaft be finished with a high degree of precision. Unless this requirement is met, the surface pressure of the interfitting portions would become locally high, and if its value exceeds a predetermined allowance, mounting portions would undergo plastic deformation and become wobbly or develop crack formation. The protrusion of the disk can be worked on relatively easily by machining because the protrusion is formed on the outer periphery of a rotary member. However, operation efficiency has been low and difficulties have been experienced in imparting precise finishes to the mounting grooves of the blades because it has been customary to use a formed end mill cutter in carrying out the necessary machining operations. The end mill cutter has the configuration of a rotary member that can be obtained by rotating a groove of the Christmas tree shape about its center line and is provided with cutting edges at its outer peripheral surface. When this cutter is rotated to form a groove by machining, it has been impossible to obtain optimum machining speeds at all the portions of the groove because the machining speeds vary from a portion of the cutter of large diameter to a portion thereof of small diameter.
When the mounting groove of the blade is formed by machining, the cutter is moved in an arcuate form in the direction of the groove while being rotated as it passes through the groove. Thus, it is necessary to perform a machining operation in opposite directions at left and right portions of the cutter or upwardly and downwardly. This makes the reaction of the machine oriented in opposite directions, so tha vibration tends to be produced and it is difficult to give precise finishes to the groove with a high degree of precision.
Moreover, in a formed end mill cutter, cuts are constant in amount in a machining operation in spite of a variation in the machining speed. Thus, this cutter suffers the disadvantage that a large machining reaction acts on a minimum diameter portion of the cutter and causes rupture to occur therein.